MEMS fabrication technologies share common origins with those used by the microelectronics industry, however, these technologies are now quite different. For example, integrated circuit (IC) fabrication is a largely 2D process and topology is to be minimized or avoided, while MEMS fabrication makes use of fabrication topology to generate 3D structures. Additionally, IC fabrication separates the processes of assembly and packaging, whereas the integration of these steps is an essential component of MEMS fabrication.
Currently, MEMS devices are assembled and placed in a planar package. These packages provide the MEMS device with electrical, and in many cases optical connections or some way of interacting with the environment. In the case of optical MEMS devices, this environmental interaction typically occurs via a window. The planar package is then attached to a second level package that performs alignment and protection of the entire system. Disadvantageously, this arrangement uses a large amount of space, which is a significant problem for optical MEMS devices for use in biomedical imaging.
In most electronic devices, the molded packaging acts as an exterior barrier to protect the encased device. Other special purpose substrates and wires are used to conduct signals. For example, most electronic components are in packages which are in turn mounted on a printed circuit board (PCB). The PCB provides conductive layers for signal routing. The completed PCB is then placed in an enclosure that provides protection from the environment. Disadvantageously, this method of packaging is not efficient in terms of either area or volume, and the minimum possible device size is too large for many biomedical imaging applications. Another disadvantage is that PCBs do not lend themselves to complex geometry. Achieving alignment between multiple components may necessitate multiple boards mounted in different locations in an enclosure, further increasing the minimum possible size of a system.
Connecting a MEMS device to other components is generally more difficult than connecting electronic devices to other components. MEMS devices also tend to be much more sensitive to static and other environmental issues such as moisture and heat than solid-state devices. Disadvantageously, in many cases the presence of moving parts in a MEMS device prevents the use of standard connectivity techniques such as solder balls. Wirebonding is a frequently used technique to connect to MEMS devices, however, it suffers the disadvantage that it traditionally requires a large amount of space in which the wirebonds from the die are typically connected to pads located around the die. Furthermore, wirebonding limits the design options of the package because of the need for vertical clearance above the die for the wirebonding operation.